1. Field of the Invention
The present invention generally pertains to apparatus, systems and methods for conveying equipment in a well, and more particularly to a downhole tractor that may be used in both open and cased wells and that is adaptable when used in open holes to all types of soil conditions and formation consistencies, whether they be soft, firm, or otherwise.
2. Description of the Related Art
It is known within the oil and gas industry to use a downhole tractor to convey equipment, such as logging equipment, within a well, and that the use of a downhole tractor is often especially useful when it is desired to convey equipment in highly-deviated and horizontal well bores. Examples of downhole tractors can be found in U.S. Pat. Nos. 4,670,862 issued on Jun. 2, 1987 to Staron et al. and U.S. Pat. No. 6,089,323 issued on Jul. 18, 2000 to Newman et al. The prior downhole tractors are deficient in a variety of respects, including in the areas of operating in open holes wherein the tractor may be required to traverse a variety of soil/formation consistencies (e.g., soft, firm, rigid, etc.), and in bore holes having varying diameters and non-uniform and irregular bore profiles. Also, the prior tractors are deficient in their ability to develop the forces required to convey the tools used in open holes for formation evaluation, which are usually much heavier than the tools used in cased holes for production logging. Another area in which prior tractors are deficient is in their ability to operate while submerged in drilling fluids at high temperatures well above 150 degrees C.; this is due in part to the poor efficiency of such prior tractors in dissipating heat. As will become apparent from the following description and discussion, however, the present invention overcomes the deficiencies of the previous devices and constitutes an improved and more efficient downhole tractor.